1. Field of the Invention
The present invention relates to digital communications techniques, and particularly to a method of performing peak reduction and clipping mitigation that provides an orthogonal frequency division multiplex (OFDM) Peak-to-Average Power Ratio (PAPR) reduction method for OFDM signals using compressive sensing, and to a transceiver or other apparatus implementing the method.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) has become a popular technique due to its high spectral efficiency and robustness to fading channels. In OFDM, the total spectrum is divided into narrow parallel sub-bands, each sub-band having a bandwidth that results in minimum delay spread. An OFDM signal is typically constructed by the superposition of a large number of modulated sub-carriers. This superposition results in a signal with high Peak-to-Average Power Ratio (PAPR), which distorts the signal if the transmitter contains nonlinear components, such as high power amplifiers (HPA). The signal also suffers significant spectral spreading and in-band distortion. An intuitive solution to this problem is to back off the operating point of the HPA, which degrades the performance of the HPA, or to use linear amplifiers.
The problem of high Peak-to-Average Power Ratio (PAPR) in OFDM systems has been tackled by a variety of approaches, including coding techniques, constellation reshaping, tone-reservation, and selective mapping, to name a few. For instance, amplitude clipping can be directly applied to reduce the Peak-to-Average Power Ratio (PAPR). However, this clipping results in in-band and out-of-band distortions, which result in Symbol-Error-Rate (SER) degradation and out-of-band radiation, respectively. To counter the effect of out-of-band distortions, a filter can be applied to the clipped signal. However, this might also regenerate new peaks. Hence, amplitude clipping reduces the PAPR at the expense of quantifiable distortion.
In some techniques, symbols are mapped into code words, and extra bit(s) are added to those code words. Following that, only code words that do not result in high PAPR are chosen. This technique requires lookup tables and exhaustive search for the best code word. A simple technique, known as Selected Mapping, reduces the PAPR by generating different sets of data blocks and transmitting the one with the lowest PAPR. This is done by multiplying the initial data set with different phase sequences, and the optimal phase sequence is sent separately to the receiver as side information. A similar technique, known as the Interleaving technique, uses interleaving instead of a set of phase sequences to produce different sequences of the same data, and transmits the one with the minimum PAPR.
Some methods use extra tones to add a peak-reducing signal to the original multicarrier signal to reduce the overall PAPR. This signal can be stripped off at the receiver using the information available at the reserved tones. However, none of the foregoing techniques have proven entirely satisfactory.
Thus, a method of performing peak reduction and clipping mitigation solving the aforementioned problems is desired.